1. The Field of the Invention.
The present invention relates to an error detection system and more particularly to a liquid crystal display (LCD) error detection system for inspecting LCD display screens and detecting the faulty LCD cells.
2. Prior Art.
LCD devices are used as a display element of a variety of electrical systems. Recently, the ability to produce LCDs in pixel sized cells as part of a larger array has been achieved. This has resulted in LCD devices having tens of thousands of LCD pixel cells in a matrixed array that can perform as a computer monitor. However, the smaller size and increased number of cells creates a greater number of faulty LCD cells. Having even a small percentage of faulty cells can significantly impair the functionality of an LCD device, such as an LCD display screen.
LCD cells are well known in the art. A typical LCD cell comprises two plates having liquid crystals between them. When a sufficient voltage difference between the two plates exists, the liquid crystals organize into a structure that allows light to pass through the LCD cell. Without a voltage differential, the liquid crystals organize into a structure that makes the LCD cell opaque. Thus the relative voltage potential of the two plates determine whether the LCD cell becomes transparent.
LCD devices are well known in the art. A typical LCD device comprises rows and columns of LCD cells configured in a matrixed array. One of the two plates of every LCD cell is coupled to a common line. The second plate is coupled to a switchable voltage source. By changing the second plate's voltage the transparency of the LCD cell is changed. Manufacture of the LCD device can result in several types of errors. Some errors address the overall performance of the device, such as contrast, cross talk, purity, response time, and image memory. Other errors relate to particular errors in the LCD cell construction, identified as Types I, II, and III errors.
Contrast is the difference in light intensity between the LCD cells in their transparent state and their opaque state. Ideally, an LCD device has a uniform contrast level because all the LCD cells of the device are similarly constructed. By testing a small sample of cells the contrast level is determined. A certain minimum contrast level is necessary to allow a LCD device to perform as a computer monitor.
Cross talk errors relate to areas adjacent to opaque regions of the screen. Normally, transparent LCD cells directly adjacent to opaque cells should have as great an intensity as transparent LCD cells not adjacent to an opaque cell. However, a common problem in LCD devices is that "ghost" regions appear around the opaque region. These ghost regions are less transparent than the average transparent LCD cell and thus appear gray.
Purity is defined by the uniformness of intensity over the entire LCD device. Ideally, an LCD device should have a uniform intensity level since the LCD cells of the device are similarly constructed. However in reality some LCD devices have a substantial number of dissimilar LCD cells. When different regions have differing intensity levels, the device is impure. Absolute purity is defined as every LCD cell having the same intensity throughout the array. Although absolute purity is impossible as a practical matter, a sufficiently high purity is necessary for device performance.
Image memory refers to the phenomena of a prior image's "ghost" appearing on the screen. The image memory lies in the length of time a prior image was displayed. The voltages present on the two plates of the LCD cell cause the phenomena by altering the structure and orientation of the liquid crystals between the plates. Image memory can be identified either as faulty pixels or as impure areas, depending on the severity.
Type I LCD cell errors involve two adjacent LCD cells that have a common liquid crystal region. Improper separation of the two liquid crystal regions due to faulty construction results in a Type 1 error. A Type I error allows both LCD cells to become transparent. To correct a Type I error, the two liquid crystal regions must be separated so that a voltage difference in one cell does not affect the other LCD cell.
Type II LCD cell errors involve an LCD cell and its adjacent line being electrically coupled. Faulty construction by allowing a liquid crystal region to border an active line allows this type of error. A type II error causes the LCD cell to be permanently opaque. To correct a Type II error, the liquid crystal region must be electrically isolated from the active line.
Type III errors involve the connection of a LCD cell to an active line. This occurs because the connection has shorted out, causing the LCD cell to be permanently opaque. It is not presently practicable to repair this type of error. However, if having the LCD cell permanently transparent is preferable to having it permanently opaque, the connective line can be severed to turn the LCD cell into a permanently transparent cell.
An LCD structure and method for fault testing in LCD devices is described in U.S. Pat. No. 4,734,688 (Adams). The device in Adams has two different signals applied to each LCD cell. The first signal is the normal control signal and operates at a set frequency. The second signal is a checking signal and operates at half the frequency of the first signal. During normal operation of an LCD cell, the first signal controls the operation of the cell. This results in the cell appearing transparent when the cell is activated by a voltage difference. If the first signal stops then the cell is activated, the second signal causes the cell to be transparent at half the rate of the first signal. This results in the LCD cell appearing to blink.
There are several limitations to the device and method described in Adams. The Adams device tests only the drive signal. One problem is the device involves a combination of larger seven cell LCD character displays. The Adams device requires additional circuitry to provide two signals to each cell. The additional circuitry would require more space to install in LCD screens. Another problem is that the faulty cell must be activated by the second signal to blink. If the second signal also fails, then the faulty cell will not blink. Also, the Adams device and method does not have the ability to detect type I, II, or III errors. Furthermore, the Adams device and method only identify errors in already functioning devices.
In scanning an LCD device, aliasing may introduce errors. Aliasing refers to a phenomena that occurs when the ratio of sensors to objects (which are to be sensed) is not a whole number. Lacking a one-to-one direct correspondence a chance exists that an object will escape detection when it lies between two sensors. To avoid aliasing, the ratio of sensors to objects should be a whole number or sufficiently close.
What is needed in the art is an LCD error detection system that can inspect an LCD display screen and detect faulty LCD cells. Also what is needed is an LCD error detection system that can test for contrast, cross talk, purity, response time, and image memory of LCD display screens. One object of the present invention is to provide an error detection system to inspect and detect faulty LCD cells. Another object of the present invention is to provide a system that can test screen parameters.